The present inventor, Halbert Fischel, has been researching and developing efficient utilization of materials and processes using mechanical engineering, electrical engineering, electrochemistry and fluidics. His early work included use of advanced fluidics in association with rotating microporous membranes for use in diverse fields, including for example “blood fractionation systems”, and “hemodialysis systems”, as disclosed respectively in U.S. Pat. No. 5,034,135 of 23 Jul. 1991, U.S. Pat. No. 5,376,263 of 27 Dec. 1994, and U.S. Pat. No. 4,366,051 of 28 Dec. 1982. More recently the inventor has been focusing upon advanced fluidics integrated with electrochemical cells, resulting in a series of ten issued patents, so far. Each of those ten patents is associated with a “Case” letter (identified below), and each is incorporated herein by reference thereto. This Non-Provisional application, identified as Case M, is therefore related to the following patents and patent application of the present inventor, Halbert P. Fischel.
Case A: “Electrochemical Cells Utilizing Taylor Vortex Flows”, application Ser. No. 12/800,658 of 20 May 2010, now U.S. Pat. No. 8,017,261 of 13 Sep. 2011.
Case AI: “Electrochemical Cells Utilizing Taylor Vortex Flows”, application Ser. No. 13/194,049 of 29 Jul. 2011, now U.S. Pat. No. 8,283,062 of 9 Oct. 2012, which is a division of application Ser. No. 12/800,658 (Case A): now U.S. Pat. No. 8,017,261 of 13 Sep. 2011.
Case A2: “Galvanic Electrochemical Cells Utilizing Taylor Vortex Flows”, application Ser. No. 13/235,480 of 18 Sep. 2011, now U.S. Pat. No. 8,187,737 of 29 May 2012, which is a continuation-in-part of application Ser. No. 13/194,049 (Case A1), now U.S. Pat. No. 8,283,062.
Case B: “Fuel Reformers Utilizing Taylor Vortex Flows”, application Ser. No. 12/800,710 of 20 May 2010, now U.S. Pat. No. 8,187,560 of 29 May 2012.
Case C: “Chemical Process Accelerator Systems Comprising Taylor Vortex Flows”, application Ser. No. 12/800,657 of 20 May 2010, now U.S. Pat. No. 8,147,767 of 3 Apr. 2012.
Case D: “Direct Reaction Fuel Cells Utilizing Taylor Vortex Flows”, application Ser. No. 12/800,672 of 20 May 2010, now U.S. Pat. No. 7,972,747 of 5 Jul. 2011.
Case E: “Dynamic Accelerated Reaction Batteries”, application Ser. No. 12/800,709 of 20 May 2010 with additional inventors Philip Michael Lubin and Daniel Timothy Lubin, now U.S. Pat. No. 7,964,301 of 21 Jun. 2011.
Case F1: “Cross-Flow Electrochemical Batteries”, application Ser. No. 13/171,080 of 28 Jun. 2011, now U.S. Pat. No. 8,158,277 of 17 Apr. 2012, claiming benefit of U.S. Provisional Patent Application No. 61/388,359 filed 30 Sep. 2010, and of International Patent Application No. PCT/US 10/39885 filed 25 Jun. 2010.
Case G: “Thick Electrode Direct Reaction Fuel Cells Utilizing Cross Flows and Taylor Vortex Flows”, application Ser. No. 13/174,686 of 30 Jun. 2011, now U.S. Pat. No. 8,124,296 of 28 Feb. 2012, claiming benefit of U.S. Provisional Patent Application No. 61/388,359 filed 30 Sep. 2010.
Case H: “Galvanic Electrochemical Cells for Generating Alternating Current Electricity”, with inventor Sheldon Epstein, application Ser. No. 13/437,771, now U.S. Pat. No. 8,394,518 of 12 Mar. 2013.
Case L: “Electrodes for Electrochemical Cells”, U.S. Pat. No. 9,337,474, of which the present application is a continuation-in-part application thereof.
Case O: “Ultralow Resistance Electrodes For Electrochemical Cells” Ser. No. 15/274,476 filed 23 Sep. 2016, of which the present application is a continuation-in-part of U.S. Pat. No. 9,337,474 having U.S. patent application Ser. No. 14/717,139.
Case M: “Electrochemical Cells With Convection,” Ser. No. 62/286,994 filed Jan. 26, 2016.
The above patents are incorporated herein by reference thereto.
This invention relates to galvanic electrochemical cells that produce direct current electricity from stored energy in cells that employ mobile electrolyte. Rechargeable (secondary) and air/oxygen breathing convection batteries and fuel cells are disclosed examples.
(Ω-cm2 is resistance, R of 1 square centimeter of electrode area in ohms. μm is micron)
Mobile electrolyte is not commonly employed in secondary batteries and fuel cells although examples can be found in the patent and scientific literature. Theoretically, electrolyte flow can move ions from one electrode to the other much faster than diffusion. Such ion exchange between polar opposite electrodes is fundamental in electrochemical cells that produce direct current electricity from stored energy. That ion exchange rate limits electrical current. Redox chemical reaction rate at the faradaic solid surface interface with electrolyte is another electrical current density limitation. Mobile electrolyte convection greatly speeds both aspects of ion kinetics. It was used in Apollo Mission fuel cells and replaced by less efficient solid state and porous matrix electrolyte versions because multiple series connected mobile electrolyte cells yield high voltage but discharged through electrolyte circulation pathways.
Batteries suited for common use do not employ convection because electrode permeating electrolyte washes out unstable electrode materials which also offer too much resistance to liquid flow. U.S. Pat. No. 8,911,893 B2 to Suppes provides an example of a packed bed electrode permeated by electrolyte providing unit cell voltage. Compression of the cell is applied to keep active particles and conducting additive (carbon black) immobilized and to somewhat improve electrical conduction. Volume energy storage capacity is not higher than common standard paste applications and there is no suitable provision for multiple cell series-connected electrolyte circulation. When multiple cells are wired in series, each cell requires its own circulation pump or similar discharge through electrolyte circulation pathways will occur as described for Apollo Mission fuel cells. That was the principal reason for the historic move to fixed electrolyte.
Battery electrode/electrolyte architecture employs a 2-phase liquid/solid relationship between active solid faradaic (electronegative anodic and less electronegative or electropositive cathodic) surfaces and attached liquid (dissociated ionic) electrolyte. Energy storage is a function of the mass of faradaic material having surface exposed to electrolyte and capable of redox valence change. Energy storage being the principal function of the battery, electrode and associated battery architecture must optimize faradaic material volume fraction. That said, energy can only be extracted if electrical current flows to the battery terminals. Prior art packs faradaic material in carbon black powder which serves as the conductor carrying electricity to metal. Application of combined carbon and active faradaics is limited to low concentration of carbon and thin layers on metal surfaces because of high (10 Ω-cm2) electrical resistance in the applied electrode layer. The high resistance, consequent limitation of thin layer energy storage and low area specific current density is compensated by large surface area to produce acceptable net current (ampere-hour Ah) rating known as C-rating. What is less widely appreciated is that area specific current density, i (A/cm2) controls recharge interval.
It is an electrochemical given that electrode current density controls time to recharge the stored energy on an electrode surface defined by the active faradaic mass per unit area of electrode. It is also understood that electrode current density is limited because electrode resistance produces a voltage decrement given by iR and i2R heating is a further serious issue. One fundamental purpose of this invention is to show how to significantly reduce R and to do it without compromising area specific energy density. It is a further purpose to provide mobile electrolyte between electrodes to support the higher electrical current with concomitantly high ion current required in any battery to balance electrical current. It is a further purpose to provide novel, unprecedented electrolyte circulation architecture for multiple series connected cells for high voltage that will prevent discharge through electrolyte circulation pathways.
All fuel cells fundamentally involve convection as the only way to bring fuel and oxygen to the 3-phase liquid/solid/gas redox reaction required on the surface of solid catalyst even as in most cases, electrolyte is not liquid. Many fuels are in liquid form and reaction products invariably carry water steam. Solid electrolytes, e.g. ionomeric polymers require low temperature and noble metal catalyst. Solid oxide electrolyte operates at the other end of the temperature scale and remains problematical as to size and cost. Both react at a triple-phase conductor/ion/gas boundary. Molten carbonate and many alkali electrolytes function effectively at intermediate temperature using inexpensive catalyst as effectively as noble metals. While electrode resistance remains an issue in all these examples, energy is stored in fuel and air so prior art has increased the ratio of electron conducting structure to catalyst mass in most cases. Specific electrode area resistance has reached ≈½ Ω-cm2 with current density ≈¼ to ½ A/cm2. Most fuel cell chemistry is based upon oxidizing H2 whether as such or extracted from hydrocarbon fuel. Consequently, ≈1 volt is all that can be produced. ½ ohm times ½ amp is ¼ volt or a 25% decrement in output voltage. It is also a ⅛ watt/cm2 heat load or ⅓rd of the power for a cell delivering only ⅜ watt/cm2.
It is physically possible to greatly increase catalyst concentration by volume compared to present electrode formulations. That would increase exchange current density, io with less voltage decrement, η in accordance with the Butler-Volmer equation, η≈k ln(i/io) where k is a proportionality factor. This is not done in prior art because the voltage decrement iR would defeat the purpose. The usual noble metal catalyst volumetric proportion is less than 1%. It can easily be increased by more than two orders of magnitude whereas Pt has been reduced from 28 to 0.2 mg/cm2 of electrode area in common practice since electrode current density, <100 mA/cm2 remains so low. R in this invention is reduced by several orders of magnitude by making electrodes in accordance with U.S. Pat. No. 9,337,474 et seq. and incorporating convection to accelerate inter-electrode ion exchange. Thus, io is increased in proportion to increased catalyst mass density and it follows that i is as well.
Inter-electrode electrolyte convection is no longer found in prior art fuel cell technology. The present inventor has incorporated Taylor Vortex Flow (TVF) in since issued patents listed hereinabove and incorporated herein by reference. This invention makes electrodes porous and freely permeable to electrolyte so that a single circulation flow path becomes possible through both electrodes and the gap that separates them, now for the first time in fuel cell architecture. It is done in such a novel way that many cells can be electrically connected in series for high voltage using a single electrolyte circulation path and pump.
The desired purpose of the fuel cell is to be able to increase power density in proportion to fuel supply rate. In prior art PEMFC, even if all factors referred to above are mitigated, there remains the ion selective electrode separating membrane e.g. (Nafion™) having an order of magnitude more cation permeability in the plane of the membrane than through it where it is actually needed. It does more than prevent inter electrode convection. It also limits cell temperature to 60° C. At higher operating temperature it tends to degrade more rapidly which accounts for the noble metal catalyst requirement. Eliminating a membrane entirely in favor of a freely permeable filter or nothing at all to impede inter electrode ion exchange convection will greatly enhance power density. More cost effective catalyst can operate at 300 or more ° C. with nearly the same or better activation as the noble metals. High temperature will also enable direct oxidation of most hydrocarbon fuels for better energy conversion efficiency than presently possible with low temperature fuel cells.
Electrodes commonly used in electrochemical cells, e.g. battery and fuel cell, etc. cannot be used in this invention, either at all or without significant modification. All share the same defect namely, too much electrical resistance. Pastes impose ½ Ω-cm2 on fuel cell and 10 Ω-cm2 on secondary battery current flow; little improved by conduction additives and binders. Emerging technology has used carbon nanotubes (single wall, SWCNT and multiwall, MWCNT, collectively referred to here as CNT) and some other forms of graphene, e.g. sheets attached as deposits on metal or precipitated as nonwoven cohesive mats, i.e. pellicles. Prior art has not succeeded in attaching any of these materials to metal electrodes with less resistance than ½ Ω-cm2; the main source being contact resistance between CNT material and the metal surface, Asari[24]. Carbon structures attached as deposits on metal tend to peel off when exposed to charge/discharge cycles in fixed electrolyte; which testifies to their feeble electrical connection even when ‘attached’. One can imagine what would happen when electrolyte flows on or through the material. Gold coatings were used by Nano-Lab, www.nano-lab.com/buckypaper to improve electrical contact of cohesive non-woven CNT with metal but reported 0.1 Ω-cm2, nevertheless.
Convection electrodes have been reported, Suppes, using packed beds of faradaic particles under 1,000 kPa compression. In order for concentrated electrochemical activity to occur the particles must be very small, e.g. nanoscale or at least of micron dimension. That leaves pathways too narrow and tortuous for achieving practical electrolyte permeability. Suppes reports much less than 1 Darcy with very little alternative to improve on that value. A reasonable channel pathway flow cross-section area would have to be ≥25 μm2 in order to achieve at least 2 Darcy permeability for 1 centipoise (cp) electrolyte. Electrodes of this invention achieve ≥20 Darcy permeability and electrical resistance <10−3 Ω-cm2.
Furthermore, novel architecture is introduced to eliminate voltage loss in series connected mobile electrolyte cells due to reverse electro-motive force, emf imposed by electrolyte circulation pathways used in banks of prior art unit cells. A unit cell is defined as comprising a single anode-cathode pair producing unit cell voltage which is generally too low to be useful, especially in high power applications. Mobile electrolyte can greatly accelerate ion mobility and kinetics in secondary batteries for faster charge and discharge cycling but is rarely seen in prior art. Mobile electrolyte is not used in capacitors because ions barely move away from the surfaces where they are created but is an essential feature, even in putatively solid state fuel cells to deliver oxygen and fuels and remove reaction products. Mobile electrolyte is fundamental to the way flow batteries operate and water electrolysis relies on mobile electrolyte to add water and remove separated gases.
The problem with prior art electrolyte circulation was evident in mobile electrolyte fuel cells used in the Apollo lunar lander. Multiple series connected fuel cells needed separate pumps and circulation loops to prevent, e.g. high voltage anodes from communicating with low voltage cathodes through an electrolyte path where a back emf circuit is closed through the load. This invention solves the problem for the first time with unique series connected electrode architecture wherein anodes are indirectly exposed to one common electrolyte pool and cathodes to a separate common electrolyte pool. Circulation between electrolyte pools is by way of a single circulation loop across a gas break or other device separating inlet and outlet electrolyte. The new feature is the inability of anodes at various electrode potentials, but insulated from one another or cathodes in the same condition to close an electrical or ion kinetic circuit that would otherwise allow them to discharge through their respective separate electrolyte pools.
Advantages of electrolyte convection in this invention include automatic internal cooling and heat balance for batteries and fuel cells. Battery self-discharge and electrochemical cell corrosion can be virtually eliminated by emptying the cell when not in use or drawing current. Also, this invention does not require separators, filters or expensive ion selective membranes to prevent short circuit between polar electrodes.
Galvanic materials lodged within the electrodes of this invention cannot be dislodged or otherwise lost to convective electrolyte because they are configured in accordance with issued U.S. Pat. No. 9,337,474 to the inventor of this application. Suppes uses a packed bed of particles that must be retained by filtering membranes or they will most certainly wash out. A nontrivial issue is the resistance to electrolyte flow offered by any micro porous structure such as packed beds of micron or nano scale particles with permeability significantly less than 2 Darcy. This invention can use the most advanced or emerging nanoscale galvanic material for their associated high specific electrochemical activity in electrodes having unprecedented permeability not less than 2 and often greater than 20 Darcy. Common practice reduces electrolyte flow resistance, if at all, by using larger pores and particles or shortening fluid path through the electrode. Prior art teaches incompatibility of low flow resistance and high electrolyte convection velocity flow through nano or micro scale structures and therefore, teaches against this invention.
There have not been many important examples of convection in electrochemical cells in actual use for a rather significant reason. Heretofore, electrical current density (amp/cm2) in electrochemical cells that produce direct current electricity from stored energy has been limited to very small electrode area specific current density commonly in the range of 0.01 amp/cm2 (batteries) to ½ amp/cm2 (fuel cells). That is a consequence of high ohmic resistance between the active galvanic surfaces producing current by virtue of redox chemical reactions taking place on such surfaces and metal as the current collector. The inventor has measured the resistance of many samples and finds it consistent with literature that deals with the subject. That means 1 amp/cm2 will produce a loss of ¼ to ½ volt in fuel cells that barely generate 1 volt. In Li-ion batteries 10 Ω-cm2 resistance can tolerate little more than 0.008 amp/cm2 especially during the recharge cycle or excessive heat can cause hazardous conditions. The relationship accounts for low current density being accepted as a universal barrier. In popular examples 3.25 Ah capacity over 200 cm2 area needs ≈0.01 A/cm2 or 112 minutes to absorb a 90% charge. Only higher current density at much lower electrode resistance can shorten the recharge cycle.
Low practical current density also accounts for acceptance of diffusion as the ion kinetic mechanism in electrochemical cells even though electrolyte convection at 1 cm/sec is 103 times faster, Bagotsky[2]. Convection accelerates ion transfer between polar electrodes required to support electrical current in batteries and fuel cells. But, if current density is otherwise limited there is little need for convection. The inventor has reduced ohmic resistance to less than 1 milliohm-cm2 which clearly translates into the possibility of very much higher current density and concomitant power in electrochemical cells. Therefore, the need and interest in electrolyte convection follows.
The term, ‘galvanic material’ is referred to hereinafter and defined as containing carbon (graphene) nanostructures comprising at least one of; carbon nanotubes (CNT) and graphene sheets (GS) having surfaces being at least one of; uncoated, permanently coated with and adjacent, particles comprising one of; catalyst and chemically reactive material that participates in redox chemical reactions that store and release direct current electrical energy. Hereinafter, electrochemical redox active materials are one of; generic faradaic (battery) and catalytic (fuel cell) galvanic materials and used in this invention to comprise structures now identified and defined. Porous galvanic ‘membranes’ comprise cohesive galvanic material having an uncompressed thickness dimension defined as the average length of vectors normal to and extending from a membrane's first surface to where it intersects the membrane's opposed second surface. The membrane thickness dimension is uniform, meaning it does not vary by more than 10%. Cohesive shall mean the membrane possesses tensile strength not less than 0.1 MPa even though CNT and GS of which it is comprised has individual tensile strength greater than 60 GPa which is more than any other material known. Freestanding cohesive galvanic membranes are often referred to as ‘Buckypaper’. Such galvanic material attached to or deposited on metal surfaces when formed as layers thereon are also common as electrodes in the art.
In this invention, galvanic material is not painted on metal surfaces as coatings or pastes containing conductive additives and binders, common in prior art. Rather, in one embodiment, a metal sheet has one surface covered with a plurality of spaced apart metal surface prominences of substantially equal height and minimum enclosing circle diameter. The prominences can be defined as metal finger-like projections and will be referred to hereinafter as spaced apart metal villi extending vertically outward from one surface of the metal sheet, FIG. 1. Any location not at a prominence cannot be further from a nearest prominence than three times the thickness dimension of the galvanic membrane. The sum of all villi enclosing circle areas shall be less than 10% of the metal sheet area, with minimum circle diameter not less than defined membrane thickness. Villi height shall be not more than 10% greater than the membrane thickness dimension. The side of the metal sheet bearing prominences shall compress a membrane against the plain surface of a second metal sheet but villi may not cut or penetrate the membrane. Sustained static compressive force shall be sufficient to cause the electrical resistance between the two metal sheets, as measured by a standard ohmmeter, to be less than 10−6 Ω-cm2; essentially a short circuit. The applied pressure required will normally be at least 7,000 kPa. It was shown in Fischel U.S. Pat. No. 9,337,474 that such pressure and consequent contact resistance between membrane compression points and a metal surface produces a net functional electrical resistance between active galvanic material anywhere within the membrane and the metal of less than 10−3 Ω-cm2.
Pressure is applied to and resistance measured between the outer second surfaces of first and second metal sheets as detailed hereinabove. First and second metal sheets and galvanic membrane must have common area dimensions and at least two distinct edges to accommodate electrolyte flow entry and exit. Villi may not have sharp corners or edges so as to not cut or penetrate the membrane. The described structure and pressure under which it functions defines the minimal basic electrode of this invention shown in cross-section in FIG. 2. The 3-layer sandwich can be repeated and stacked to further comprise an electrode of this invention. It is noted that electrolyte can enter an electrode at a metal sheet edge, flow within the electrode over the uncompressed portion of membrane surface and exit at a separate metal sheet edge.
In a further embodiment of electrodes of the invention both metal sheets have matching villi compressing the membrane from its opposite surfaces. The villi of both sheets are aligned so the membrane is compressed between opposing villi. Villi height is ≈10% greater than ½ membrane thickness whereby electrolyte flows over both uncompressed surfaces of the membrane.
In a further embodiment of this structure shown in cross-section in FIG. 3, a first metal sheet is blocked at an electrolyte entry edge and open at its corresponding electrolyte exit edge. The second metal sheet is open at the same electrolyte entry edge and blocked at the electrolyte exit edge. It is a manifold rubric that forces electrolyte to flow through the membrane before it can exit the electrode. Compression causes functional electrode resistance of <10−3 Ω-cm2 when measured resistance between first and second sheet outer second surfaces is <10−6 Ω-cm2.
In another embodiment of electrodes of the invention a plain square weave metal wire screen, FIG. 4, covers and is adjacent the smooth first surface of metal sheet, FIG. 5. Wire crossover locations create localized prominences serving the same function as above described for metal sheet villi. Plain wire weave defines a square pattern having window side length, here not more than 4.25 times membrane thickness, which assures the same compression criteria stated above whereby any uncompressed location is no more than 3 times membrane thickness from a nearest compressed location. Wire diameter is ≈5% greater than ½ membrane thickness when the screen compresses the membrane against a smooth metal surface. This provision can also assure no more than 10% of the membrane area is compressed. A second smooth metal sheet having a first surface covered by and adjacent the first surface of the galvanic membrane is used to sustain compression of the metal screen and membrane such that electrical resistance between compression sheets is <106 Ω-cm2. Electrolyte enters the electrode at an edge and flows over the membrane uncompressed surface to exit at a separate edge.
In another embodiment of electrodes of the invention, a single membrane is compressed between two metal wire screens as hereinabove defined and necessarily positioned so that all windows of each screen align in vertical projection, referenced to a tangent plane externally adjacent all screen wire crossovers. Two metal sheets compress their adjacent metal screens whereby electrical resistance between compression sheets is <10−6 Ω-cm2. If all screens are not vertically aligned as now above defined, compression will collapse the stack. In a further embodiment of this structure one metal screen, FIG. 6 is blocked at a defined electrolyte entry edge and open at a corresponding electrolyte exit edge. The second metal screen is open at a same electrolyte entry edge and blocked at the same electrolyte exit edge. It is again a manifold rubric that forces the electrolyte to flow through the membrane before it can exit the electrode.
It is possible to coat the opposing tops of villi, FIGS. 2 and 3 or screen wire crossover locations, FIGS. 5 and 6 in brazing or solder so that hot compression will permanently accomplish what sustained compression is meant to do. Galvanic membrane material is stable to 1150° C. so melted solder/brazing will electrically join contact points without damaging the membrane. Solder/brazing also leaves a permanent physical connection at contact points whereby sustained compression of the assembly is no longer required.
FIGS. 7a & 7b illustrate sectional views of a stack of alternate wire screen and membrane. The stack is secured under sustained or hot brazed compression. Section lines are shown in FIG. 4. FIG. 7 is FIG. 6 with many repeated membranes secured between vertically aligned metal wire screen windows. FIG. 8 illustrates how the FIG. 7 stack side-ports are arranged to force electrolyte entering the stack from the left to flow through every membrane, to enter flow paths surrounding each membrane and leave the electrode on the right. Screens are numbered n to n+1 from bottom. Even numbered left side entry ports remain open. Odd numbered on the left are blocked. Open exit ports on the right are odd numbered. Even numbered right-side ports remain blocked. Selected screen edge closure forces normal vector permeation of electrolyte through active membrane layers secured by one of; closely spaced multipoint sustained compression and braze at facing screen crossover locations. There are n+1 screens for n membrane layers. Resistance to flow is normal vector permeability through a single membrane since surrounding flow spaces offer negligible obstruction. It should be understood that what is depicted is only one electrode; either anode or cathode, not both. Various aspects and embodiments of the invention can be summarized as follows:
An electrode for an electrochemical cell, the electrode comprising:                a) One of; 1 and n>1 galvanic membrane(s) having a thickness dimension defined by the average length of vectors normal to the membrane(s) first surface and extending to where they intersect the membrane(s) uncompressed second surface and;        b) A first non-porous metal sheet having first and second surfaces and;        c) A second non-porous dielectric sheet having first and second surfaces and;        d) One of; 1, n and n+1 plain weave metal wire screen(s) with wire diameter not less than one half the galvanic membrane(s) defined thickness dimension and;        e) Wherein, uniform wire mesh spacing is not greater than 4.25 times the galvanic membrane(s) defined thickness dimension and;        f) Wherein, all galvanic membranes lie between a metal wire screen adjacent each of its first and second surfaces in a stack of membrane(s) and screens and;        g) Wherein, a metal wire screen is adjacent at least the first surface of the second non-porous sheet and;        h) Wherein, the second surfaces of the non-porous sheets are subjected to a sustained compressive force of at least 7 million Pascal (7 MPa≈1,000 psi) and;        i) Wherein, metal wire screens are collectively in incompressible vertical alignment.And:An electrode for an electrochemical cell wherein;        Brazed joints at wire screen crossover locations replace sustained compression.And:An electrode for an electrochemical cell wherein;        One of; sustained compressive force and brazing at wire screen crossover locations produces electrical resistance between first sheet and metal screen adjacent the second sheet of not greater than 1 μ-ohm.And:An electrode for an electrochemical cell wherein:        a) All sheets, membrane(s) and wire screens share common area dimensions and;        b) Area dimensions define at least first and second distinct edges and;        c) Screen number 1 adjacent the first surface of the second sheet and every subsequent odd numbered screen in the stack is blocked to electrolyte entry at the defined first edge and open for electrolyte exit at the defined second edge and;        d) Screen number 2 and every subsequent even numbered screen in the stack is open for electrolyte entry at the defined first edge and blocked to electrolyte exit at the defined second edge.        
FIG. 10 is the equivalent of FIG. 8 using 2-sided matching metal villiform screen one of which is shown in FIG. 9.
It will be understood that the galvanic membrane comprising at least one of; CNT, GS and mixtures thereof contain high concentrations of graphene molecular structure surface area. The membrane volume fraction occupied by carbon surface structure depends on the application. Battery electrodes minimally need ≥2 weight-% to allow for maximum galvanic active material volume. A 100 μm thick membrane can contain 800 cm2 of active surface per cm2 electrode area. On nano-coated surfaces used in fuel cells and ultrahigh capacity asymmetric pseudosupercapacitors >30% volume provides >2,500 cm2 in a 100 μm thick membrane. The stack of galvanic membrane and metal wire screen shown in FIG. 8 is a single composite electrode having extraordinary concentration of redox active mass and surface area connected to cell terminals with unprecedented low electrical resistance.
The discovery that galvanic particles not otherwise attachable to CNT or GS surfaces as a lodged and immobilized coating become equally lodged and immobilized within galvanic membranes of this invention when membranes are incorporated as hereinabove described in a composite electrode under pressure was disclosed in U.S. Pat. No. 9,337,474 and co-pending cases on file. Absent closely spaced multipoint compression of the membrane, particles not securely attached to graphene surfaces will wash out when subjected to convection of mobile electrolyte. It is a novel and crucial feature of this invention for specific purposes.
The preferred embodiment for electrodes of this invention is according to FIG. 8 using metal wire plain weave screen because it is readily available and very economical. 55 μm wire in 60 mesh plain weave (inverse of wire spacing in inches) can use 100 μm thick membrane. 177.5 μm (7.1 mil) wire works with a 350 μm thick membrane and 17 mesh. There is 20 μm stainless wire to accommodate 38 μm thick membrane (about a lower limit for this configuration) and 160 mesh. For all these practical examples the fraction of membrane area compressed and related wire volume is less than 5%. If flow channels occupy <10% of electrode volume to ensure negligible flow restriction, about 85% remains for active galvanic membrane volume. That is an important metric in electrochemical cell design.
Electrolyte flow resistance for electrodes of this invention in accordance with FIG. 8 is not more than that of a single membrane. FIG. 11 provides some examples. Resistance to flow over membranes is negligible but less important because it requires ion diffusion through the membrane which slows the electrochemical process. When electrolyte can flow intimately over every active particle, diffusion is a negligible factor in ion exchange kinetics. Diffusion time constant varies inversely with diffusion path length squared. For galvanic membranes suitable for use in this invention permeability is at least 2 Darcy and generally greater than 20 Darcy as proven by the data of FIG. 11.
Another transformative feature of this unique convection electrode architecture is the fact that effective convection takes place within an ultrathin extremely large area electrode. In prior art, porous convection electrodes are thick in order to hold a suitable mass of active galvanic material. There the electrochemical process begins in a thin electrolyte entry zone and works its way toward the far end of the electrode whereby electrolyte must pass through a considerable length of spent material. The same defect can apply to electrodes of FIG. 7b if only configured for electrolyte flowing over a membrane. FIG. 8 architecture provides thin convection electrodes where their effective area is one layer area multiplied by the number of layers in the stack. It is superior to all examples in known prior art. Furthermore, each layer is connected to cell terminals with ultralow ohmic resistance so their area specific current density can be more than two orders of magnitude greater than what is now possible with electrodes in any prior art application. The effective electrode area is multiplied by the number of layers in the stack so net current level possible with the composite electrode and therefore power density becomes positively transformative.
A unit cell is one anode and one cathode paired and configured to allow electrolyte to flow as described above sequentially through both. Anode is differentiated from cathode by the galvanic materials contained within their respective membranes. FIG. 12 shows one electrolyte recirculation loop through two composite electrode stacks. Flow direction is arbitrary and reversible. The flow path contains a pump and electrolyte reservoir with gas dividing the flow path to prevent discharge by way of electrolysis in the flow loop caused by the potential difference between anode and cathode. The composite electrodes could be spaced apart 4-sided stacks but are conveniently shown as concentric annular stacks.
An annulus has two edges; one each for electrolyte entry and exit as previously described hereinabove. Additional edges need to be sealed and merely add unnecessary complexity. An inner annular composite electrode stack contains a central lumen suited to electrolyte access and outer cylindrical surface. An outer annular composite electrode stack defines a cylindrical inner surface spaced apart from the outer cylindrical surface of the inner stack. Radii of each stack are configured to create the cylindrical annular gap between facing surfaces of the composite electrodes. The outer annular composite electrode stack defines a cylindrical outer surface as one surface of a chamber fitted to collect electrolyte for circulation. This unit cell electrode topology is not unique in electrochemical art although not much used in convection electrochemistry. It is convenient for what follows.
The unit cell of FIG. 12 is capable of inordinate current at virtually negligible resistance delivered to its polar terminals. The following invention discloses how high voltage can balance amperage for optimum practical power density. FIG. 13 depicts several unit cell stacks, each being as described for FIG. 12. They are separated by a plate containing a metal conductor carrying current between an anode of a first (lowest) unit cell to the cathode of the next adjacent unit cell above. The separating plate also contains two insulating sections that prevent the cathode of the first (lower) unit cell from shorting to the anode of the next unit cell above it. The sequence can be repeated in the same fashion for as many unit cells in a stack as needed to produce a desired voltage. The end plates provide anode and cathode terminals and intermediate plates seal off electrolyte in the gap of one cell from mixing or even ‘seeing’ electrolyte in the gap of an adjacent cell. Isolation of electrolyte in the gap of each unit cell is essential to prevent discharge through electrolyte pathways. Note the inner lumen communicates only with anodes and the outer chamber communicates only with cathodes. That feature, additional electrolyte shielding as further described below and the unit cell separating plates comprise a novel invention for the following reasons.
Self-discharge between electrodes through connected electrolyte pathways at different levels using simplified single pump circulation has discouraged prior art application of convective mobile electrolyte in spite of its known advantages. In series connection there is a sequence in the line of unit cell wiring where a single anode is electrically connected to a single cathode so either can function as the other. If a path can be traced through electrolyte connecting any such dual identity pair that exist at different potentials it is like dipping the terminals of a battery into electrolyte. Whether or not the composite cell is connected to a load, it can discharge through the electrolyte pathway even though that path does not conduct electricity. The migration of ions within the cell functions as a reverse charging process. When connected to a load it represents a parasitic current opposing the one serving the load.
Having identified such a parasitic electrolyte pathway there are two factors to consider; 1) the electric field between electrode pairs at different potentials and 2) the migration (flow) of ions in directions trying to cancel inter electrode ion exchange between unit cell electrodes that supports current to a load. The electric field tries to move anions and cations to the wrong electrodes. They actually ‘flow’ by diffusion in opposite directions, both antithetical. An electric field can be blocked by metal insulated from polarity and diffusion by opposed convection. Both are provided by this invention in unique ways as seen in FIG. 13. Anodes and cathodes are arranged, respectively, to face separate anode and cathode pools of electrolyte and are ‘screened’ from those pools by actual fine weave metal screen or a metal wall penetrated by one of; one and a plurality of small holes; with either screen or wall being supported by dielectric insulation. The holes or screen mesh is configured to cause at least 1 cm/sec convection velocity of electrolyte flow through the screen or holes as a result of forced electrolyte circulation through the composite cell. That convection velocity overwhelms the diffusion of ions trying to migrate to electrodes from the respective common pools of electrolyte shared by all anodes and all cathodes. With fully isolated electrodes, a single pump, circulation loop and electrolyte reservoir containing gas is sufficient to service a large number of series connected unit cells with voltage multiplication in either a convection battery or fuel cell without self-discharge through electrolyte circulation pathways. The latter defect has prevented using electrolyte convection to its known advantage for accelerating redox chemical reactions in prior art. In FIGS. 12 &13 there is no membrane required in the gap between positive and negative electrodes. Flow channels are macroscale for electrode permeability of at least 20 Darcy when electrolyte flows over thin galvanic membranes. It is approximately 2 Darcy when electrolyte flows through thick galvanic membranes for better electrochemical efficiency, FIG. 11. Both require convection velocity of at least 1 cm/sec for high current density that depends upon fast inter-electrode ion exchange. Faster convection velocities are practical in this invention because electrolyte convection resistance is so low. Bagotsky teaches that 1 cm/sec electrolyte convection velocity will move ions 1000 times faster than diffusion in electrolyte. That is the primary motivation for electrolyte convection in fuel cells and batteries. Further and in stark contradiction to previous practice macro channels rather than nano scale pores or channels are required here to retain nanoscale galvanic materials. For point of reference 2 Darcy means the pressure gradient across a 1 cm thick electrode is ½ atm or about 7.5 psi using 1 centipoise viscosity at 1 cm/sec convection velocity. In this invention, it is only necessary to use galvanic membranes much thinner than 1 cm and all in parallel rather than sequence. That is why electrolyte circulation can be very fast and current density higher than has ever been achieved in prior art. When electrical resistance is less than 10−3 ohm-cm2, 10 amps/cm2 can be produced in a fuel cell or 1 amp/cm2 in a rechargeable battery. Such combination of energy storage capacity and power density is positively transformational.
Compression of a cohesive galvanic membrane between opposed metal prominences with >700 Newton/cm2 force is actually >14,000 Newton/cm2 because of a pressure multiplier due to only <5% of the membrane being compressed. That pressure reduces electrical contact resistance at compressed locations virtually to 0 ohms and prevents the pellicle from coming off of the metal regardless of multiple expansions and contractions due to repeated charge discharge chemical reaction cycling. Compression locations are closely spaced so when material bulk resistance is taken into account, electrode resistance is less than 10−3 ohm-cm2 averaged over its surface. Prior art cell architecture creates grooves and depressions in pellicles and attached layers to accommodate material distortion and prevent peeling from a conductor in normal use; not otherwise to improve electrical conductivity to the conducting surface. Contrary to this invention, the remaining material is at least somewhat compressed by assembly in a spiral wound or plate structure. Such structures can only compress an entire layer wherein it could not function for its intended purpose if compressed to substantially lower ohmic resistance. An example of a concept that may appear similar but is radically different in accordance with this invention is shown in a micro photograph, FIG. 17. It is a cohesive pellicle removed from an electrode and somewhat wrinkled upon drying but still showing distortions in its surface. The grooved impressions are the result of pressing a square weave metal screen against the pellicle onto a metal current collector in an electrode. The wire cross points apply enough pressure to virtually eliminate contact resistance at those locations whereas between those locations the pellicle needs little, if any, electrical contact with the metal. Between compression points it can come completely off the metal backing but it remains uncompressed and electrochemically active. It has room (volume) to expand and contract with impunity. Yet, resistance between pellicle as a whole and supporting metal remains more than three orders of magnitude lower than conventional assemblies because distance between compression points is short and material bulk resistance is low. See FIGS. 15 and 16.
In this invention stack compression serves a vital purpose beyond immobilizing galvanic materials. Macro channels are low resistance fluid conduits that compress layers at a plurality of defined small areas separated by short distances which are a multiple of the thickness, e.g. 0.1 to 1 mm of a layer. >95% of the layer remains uncompressed and functions normally. Net resistance between active galvanic surfaces and metal, referred to electrode area, drops from about ½ Ω-cm2 in fuel cells or 10 Ω-cm2 in Li-ion batteries to an unprecedented level of less than 10−3 Ω-cm2 when membrane layers are compressed against a metal conductor with pressure applied to defined small areas of 7,000 to 30,000 kPa depending on current density expected. More than 100 times the current density in batteries and 10 times in fuel cells becomes possible because the voltage loss for that current is only 0.01 volt in batteries. Prior art voltage loss at that current density would exceed the voltage produced in most cases; clearly impossible.
In most cases cathodes in batteries and fuel cells suffer from the slowest reaction rate and require the greatest weight and volume of active galvanic material, whether faradaic or catalytic. Improving cathode materials is the focus of much ongoing materials research but cathode function is more easily improved as here described. It is convenient to select the inner annular electrode as the anode because it requires less volume than the cathode and volume varies as radius squared for fixed axial length. The real benefit comes from choosing the direction of electrolyte flow from inner chamber, through the inner anode electrode, through the gap and into and through the outer cathode electrode. Electrolyte passing first through the anode acquires excess cation concentration which accelerates cathode reactions according to ordinary chemical reaction kinetics. Electrical area current density referred, to cylindrical electrode faces, is proportional to the product of cation concentration and convection velocity. Battery recharge flow can but does not have to be in the reverse direction in most cases.
For a chosen electrode volume, electrolyte residence time within the electrode will be determined by radial dimension, stack height and convection velocity. The latter places a limit on maximum power for meeting load demand. These degrees of freedom are not available to stationary electrolyte and can more easily accommodate cell design to the load, especially in fuel cells using complex hydrocarbon fuels, e.g. DMFC. Residence time in the anode can be increased to virtual completion of fuel oxidation independent of convection velocity. That is important because reaction gases and unconsumed fuel can poison the cathode oxygen reduction reaction, ORR catalyst. High temperature operation further discussed below will mitigate incomplete oxidation and CO production.
Batteries using intercalation of metal cations, e.g. Li+, metal hydride (MH) or multivalent versions, e.g. Mg2+, Al3+, etc. in anodes do not alter their aprotic or aqueous electrolyte on a trip through both electrodes. Except for MH these metals can be intercalated on carbon surfaces or Si coatings of CNT surfaces. Cathodic material is usually 3 to 5 μm spinel or Chevrel phase particles that can be lodged and immobilized in cohesive CNT membranes as previously described. Convection velocity should be coordinated with available area concentration of faradaic material in the electrode for maximum power density. Batteries that alter electrolyte on a pass through either electrode, e.g. Ni/Fe, require coordination of convection velocity with, e.g. (OH)− anion concentration. In that case high power requires high alkali concentration, convection velocity and faradaic material loading.
Fuel cells of this invention operate without a fragile polymer semipermeable ion selective membrane or filter at elevated temperature. 80% KOH can be used at 300° C. and 100 psi fuel and air/O2 pressure with non-noble catalyst and only minor overvoltage. Molten carbonate at 700° C. is flowable with 3.1 cp viscosity and can be used with Ni type catalyst CNT coatings having less overvoltage than Pt. Because galvanic membranes used in this invention do not need or incorporate binders their temperature tolerance is 750° C. in air and 1150° C. otherwise. At these temperatures any hydrocarbon fuel is a candidate for direct fuel oxidation (sans H2 reformulation) in this invention.
Mobile electrolyte convection offers a unique opportunity to accelerate catalytic surface activity for anodic H or H+ adsorption required for hydrocarbon fuel oxidation, HOR as well as cathodic O2 reduction, ORR. Bockris[8] teaches that the catalyzed redox process occurs mainly and actually only effectively in a very thin layer of electrolyte attached as a meniscus film to the catalyst surface. Diffusion from the gas phase, through the liquid or ionomer electrolyte film and to the catalyst surface is slow and is only appreciable in a thin film where the diffusion length is short. For liquid fuels (e.g. methanol) dissolved in moderate temperature static liquid or solid electrolyte, reaction products accumulate on catalyst surfaces to further slow the process and polarize the electrode. Mobile electrolyte tends to depolarize the electrode but produces a liquid boundary layer that is thinned by fast convection velocity. Still it offers greater diffusion resistance than a meniscus edge.
The highest concentration of gas/liquid menisci is foam. Such could not be sustained in a fuel electrolyte mixture without convection. At a planned fuel cell operating temperature of 300° C. all useful hydrocarbon fuels are gas phase as, of course, is O2/air. Micro-foam producing spargers based upon venturi suction of gas into flowing liquid can produce stable flowable foam comprised of fuel gas and electrolyte. The device can be connected to the fuel/electrolyte inlet but is otherwise separate from the cell. Such foam flowing through permeable CNT membrane present an unprecedented interface of electrolyte menisci saturated with fuel covering nanoscale catalyst surfaces. Rapid redox reaction (Bagotsky) fills the foam with reaction gas and steam which proceeds to the gap where it crosses to the cathode. Interaction with catalyst surfaces through saturated menisci is well understood to be exceptionally efficient. Oxygen containing gas is pumped into the foam through tubes placed within the gap as shown in FIG. 14 and running the full axial length of all cells in the stack. Tubes are porous dielectric spargers made of ceramic frit or tubes with many small holes. They penetrate but are sealed to plates separating unit cells. Thus, electrolyte cannot communicate between cells. Other foam producing devices can be suitable for the stated purpose.
With further reference to FIGS. 12, 13 and 14, electrolyte exiting the cell is led through conduit tubing to an accumulator chamber containing gas. The gas is for illustration purposes to show a physical break in the electrolyte path. The break interrupts what would otherwise be a closed electrolyte loop between electrodes for a unit cell but serves other purposes as well. In a high voltage battery stack the gas volume absorbs volume changes within the battery for charge/discharge cycling, reconstituting electrolyte and holding it to drain the cell for storage. In fuel cells the gas/liquid interface is a defoamer and reservoir for maintaining pressure and releasing exhaust. One pump takes liquid electrolyte from the reservoir and returns it to the cell anode.
An entirely new benefit embodied by the electrode configuration of this invention as seen in FIG. 13, refers to a stack of a plurality (3 are shown) of unit cells connected in series to build high voltage. The unit cells are separated by a dielectric spacer to insulate adjacent anodes from one another and adjacent cathodes from one another. All inner and outer electrodes throughout a stack of repeated unit cells must be, respectively, either anodes or cathodes. All outer electrodes must be of opposite polarity to the inner electrodes. Thus, center and outer electrolyte chambers respectively see one polarity.
Adjacent unit cell faces are electrically connected within separating plates as follows: A first unit cell in a stack of N unit cells is unit cell #1 with its inner electrode connected to a terminal for connecting to a load. Every successive unit cell in the stack, including the first, has its outer electrode electrically connected to the inner electrode of the next adjacent unit cell, i.e. outer of N to inner of N+1. It is important that the inner electrode of n is connected to the outer electrode of N−1 but is NOT otherwise connected to any other electrode. The outer electrode of N is connected to a terminal for connecting to the load. All electrodes are electrically connected at only one axial end. The stack may be inverted with no other consequence than reversing the inner/outer numbering description.
A unique benefit of the above arrangement is an ability to isolate every electrode from electric field and ion kinetic communication with every other electrode except the one it faces to create battery potential and current. Metal shields, insulated from any polarity allow electrolyte convection through electrodes and common electrode collection pools with circulation of the latter through a single pump. For a single unit cell it was not especially important but when a stack of n cells is charged to N times V where V is unit cell voltage, the matter becomes important which the following example will illustrate.
The conventional series connected convection electrode stack can be represented by a longitudinal arrangement of anode/cathode unit cells oriented with anodes connected to cathodes and for comparable convection, electrolyte chambers are fixed on opposite sides of the long axis of the stack. Electrolyte enters anodes on one side of all cells collectively to leave from cathodes in similar fashion on the other side so circulation can be effected by a single pump. This is as closely analogous to the instant invention as may be possible. A defect is apparent in that each unit cell drives voltage in one direction but, where anode is series connected to cathode, an electrolyte path through either or both chambers exists where the series connection closes the circuit of a unit cell in the opposite direction. Even worse, one can draw as many opposing cells between higher voltage pathways as cells accumulating voltage in the intended direction. Longitudinal convection through the stack, even if it were possible, only makes matters worse because it aids reverse voltage more than the intended direction. The conventional solution is a separate circulation path for each series connected cell. That accounts for their absence from the literature.
Embodiments of this invention comprise composite galvanic electrodes constructed by stacking layers with ultralow resistance connection to cell terminals through immobilized cohesive galvanic membranes comprising at least one of; nonwoven single-walled carbon nanotubes; nonwoven multi-walled carbon nanotubes; a single or plurality of layers of graphene sheets; one of; single-walled and multi-walled carbon nanotubes attached to one or both sides of metal sheet or woven carbon fiber; and combinations thereof; and wherein the nanoscale surfaces of these structures are one of; coated with galvanic material and adjacent galvanic particles lodged therein and immobilized.
FIGS. 18 and 19 are SEM photographs of cohesive CNT membranes respectively as formed without and with lodged and immobilized faradaic particles. It is on the nanoscale surfaces of CNT within membranes or the particles they entrain where galvanic current is produced. In this invention they are connected with very low ohmic impedance to a metal current collector to support exceptional electrical current density.
FIG. 4 depicts a typical square weave metal screen or cloth. The screen is used because it has a pattern of closely spaced small areas where its thickness is two incompressible wire diameters. Everywhere else its thickness is 0 or one wire diameter. Wire cross locations and windows can be stacked in alignment to provide an incompressible structure with macro channels for low resistance electrolyte flow in any direction. It comprises a low cost electrode with negligible ohmic resistance connection to galvanic membranes.
A convection battery and fuel cell require alternate layers of wire screen and galvanic membranes comprising internal nanoscale components, e.g. CNT, galvanic materials, etc. They are immobilized by compression between aligned wire crossover locations of a wire screen and together comprise one layer in the stack of layers comprising the composite electrode. The ‘galvanic membrane’ must have thickness less than two wire diameters in order to leave channels for tangential electrolyte flow on both surfaces of the pellicle. Ion mass transport kinetics is a combination of diffusion within an electrolyte permeable pellicle and convection over or through it. Electrical current is essentially instantaneous at low resistance but cannot exceed ion current. Diffusion time interval is proportional to the square of diffusion path length which, in some cases, favors thinner pellicles holding less galvanic material per layer.
Galvanic membranes can be made with sufficient normal vector permeability to allow electrode layer entrance and exit port sequencing for electrolyte to take better advantage of that property. A galvanic membrane placed between two screens can be subject to normal vector permeation merely by alternating clear and blocked screen edges. Channel entrances with respect to one edge of a screen are open but sealed on the electrodes opposite surface. Thus, when electrolyte enters the electrode through open channels on one surface it is forced to flow into screen channels that do not block its ability to leave at the electrodes opposite surface. Electrolyte must flow through the pellicle into a separate screen. It leaves the electrode from the surface opposite that at which it entered. Diffusion is much accelerated because pellicle internal pathways are very short.